This marks the beginning of the "Dark Heart of Skyrim" story arc. Harrowstorm introduces two new 4-player dungeons, Icereach and Unhallowed Grave. This patch was also mainly meant to further improve game performance, with an emphasis on frame rate stability, so that there would be fewer drops in the player's framerate.
There was also some update to refreshing the patching and file manifest data to remove duplicate files and older data that was no longer needed. It would improve load times by a lot, streaming of assets, reliability of asset loads, and a reduction of space taken up by the client. Players had to completely re-download the game and couldn't just do an update. They didn't have to delete the old client, but they had to have enough space on their drive.
Also, players speaking Russian were now able to select Russian as their language in the ESO launcher settings. It was meant to introduce a full Russian language support that was planned later for 2020.
New Features & Updates
Harrowstorm DLC Game Pack
Two New Dungeons, Icereach and Unhallowed Grave, which introduced the player to the "Dark Heart of Skyrim" story arc
New Item Sets, Collectibles & Dyes, Achievements & Titles and Furnishings
Base Game
Performance Improvements, optimized multithreading implementation, optimizing processes for ressource spikes, multiple core usage, CPU optimizations
New PvP Campaigns
New Collectibles, Knight of the Circle armor and weapon outfit styles were now obtained by purchasing a Knight's Resolve or Knight's Rebuke from a War Researcher in Cyrodiil with Gladiator Proofs
Master Writ Furnishings
Introduction to Crafted Sets in Cyrodiil
Undaunted Achievement Furnishings
Experimenting with Solo-Queues for Battlegrounds, meaning that group queues were disabled and players could only queue solo